.container {
  height: 130px;
}
.box_3 {
  width: 100px;
  height: 100px;
  border-radius: 20px;
  border: 1px solid black;
  animation: bb_width 5s infinite;
}
@keyframes bb_width {
  50% {
    border-bottom-width: 15px;
  }
}